The membrane separation technology is being widely utilized for, for example, desalination of brine water such as river water and groundwater, desalination of seawater, production of ultrapure water in the semiconductor manufacturing industry, or reutilization of various industrial or domestic wastewaters, and in view of performance and cost, a polyamide-based composite membrane in the form of a flat membrane is generally used as the reverse osmosis membrane or nanofiltration membrane.
From the aspect of balance between the cost and the characteristics, a polyester-made nonwoven fabric is most often used as the support of the polyamide-based composite membrane, and a polysulfone is most commonly used as the porous material formed thereon. In the industrial production method of the polyamide composite membrane, a porous layer is first formed by a so-called wet process film-forming method where a film-forming solution prepared by dissolving a polysulfone in a solvent is cast on a support, solidified and deliquored with water, and on this polysulfone porous layer, a polyamide skin layer is formed by coating an aqueous polyfunctional amine solution and a polyfunctional acid chloride, allowing an interfacial polymerization to proceed, and the polyamide skin layer is cured and dried, whereby the polyamide composite membrane is continuously produced.
Also, as for the polyamide-based composite membrane in the form of a flat membrane, a spiral membrane element obtained by superimposing the membrane and a channel material, sealing on three sides with an adhesive resin, and winding these around a water collecting tube is most commonly used.
Accordingly, the support is required to have various characteristics and functions necessary at the production of a composite membrane and a composite membrane element, as well as to have not only a mechanical strength for reinforcing and supporting a membrane having a separation function but also characteristics such as bonding property and sealing property (sealability) of the bonded and sealed part of the element and resistance to chemicals during use.
That is, at the film formation of the porous layer, an appropriate density or air permeability not allowing generation of a defect such as pinhole and so-called strike-through of an excessively seeping film-forming solution, a uniformity thereof, and a surface smoothness are required. For example, if the density of the support is too low or not uniform, if a protrusion such as fiber-raising and extraneous material is present on the support surface, or if the surface unevenness is large, a pinhole, a crater-like recess or a defect called “Cut & Slit” as if the layer is cut or slit out with a knife may be produced in the porous layer, or a large number of air bubbles that are a latent defect may be generated at the interface between the support surface and the porous layer. If such a defect or a latent defect is produced in the porous layer, the aqueous amine solution coated at the film formation of the polyamide skin layer may be not uniformly held near the surface layer of the porous layer, and a defect may be produced in the skin layer or a dense skin layer may be not formed. Also, even if an apparently uniform skin layer is formed, the skin layer may be ruptured under pressure during use, and the desalination performance may be seriously impaired.
In the industrial film formation process of the polyamide skin layer, an interfacial polymerization is allowed to proceed on the porous layer surface in the course of conveying the support having a porous layer formed thereon under constant tension so as to prevent occurrence of curling or wrinkling, and then the support is usually taken up through a curing and drying step at 80 to 150° C. Therefore, mechanical strength at high temperatures is required.
If the mechanical strength at high temperatures is insufficient, the formed skin layer may be stretched together with the support, and a dense skin layer with high desalination performance may not be obtained, or the skin layer formed on the porous layer in a latent defective part attributable to the support may be locally ruptured.
Furthermore, at the production of a composite membrane element, appropriate density, air permeability and pore diameter, enabling an adhesive sealing resin to uniformly impregnate even the porous layer, are required.
On the other hand, in recent years, the worldwide increase in water demand and shortage of water due to depletion of water sources have given rise to an increasing need for water desalination by a membrane method and its effectiveness, and it is demanded to elevate the performance of the membrane itself, enhance the water permeability per unit volume of the membrane element, and reduce the production cost of the membrane element. In particular, the ratio of the support occupying in the material cost of the separation membrane is about ½ and highest and therefore, a thin and inexpensive support has been conventionally required.
International Publication No. 2006/068100 filed by the present inventors discloses a separation membrane support nonwoven fabric excellent in the property of preventing strike-through of a coating resin for separation membrane and having adequate heat resistance and high mechanical strength, which is produced by continuously spinning and laminating fabrics to form a three-layer structure of spunbond nonwoven fabric/meltblown nonwoven fabric/spunbond nonwoven fabric, and subjecting the structure to thermal bonding under specific conditions and then to calendering. This nonwoven fabric is less likely to cause fluffing giving rise to a membrane defect called Cut & Slit and sufficiently applicable as a support of a microfiltration membrane or an ultrafiltration membrane, nevertheless, when used as a support of a composite membrane, suffers from a problem that a local defect is readily produced and the desalination performance has dispersion.
As for the support using filaments nonwoven fabric, a separation membrane support consisting of at least two layers, where a low melting-point resin powder or a nonwoven fabric for bonding is disposed between a surface layer on which a membrane is formed, and a second layer in proximity to the surface layer, has been proposed in Japanese Unexamined Patent Publication No. 2007-275691, but softening of the low melting-point substance and in turn, strength reduction or dimensional change may occur at high temperatures in the curing and drying step at the film formation of a skin layer of the composite membrane. Also, the low melting-point resin powder or the nonwoven fabric for bonding may melt and this may inhibit uniform impregnation of the membrane with an adhesive sealing resin at the production of a spiral membrane element and impair the reliability of the sealed part.
Japanese Unexamined Patent Publication No. 2009-61373 has proposed to use, as the separation membrane support, a laminate nonwoven fabric that is composed of a filaments nonwoven fabric composed of a thermoplastic continuous filament. It is disclosed in this publication that the mechanical strength is increased and fluffing on the nonwoven fabric is reduced, by using a core-sheath type filaments nonwoven fabric containing a low melting-point component or using a low melting-point bonding substance between layers. However, due to melting and resinification of the low melting-point component, many locally-filmed sites are generated and the uniformity of the membrane is not satisfied. Furthermore, similarly to the above Japanese Unexamined Patent Publication No. 2007-275691, there is a problem of softening or melting of a low melting-point substance at high temperatures in the curing and drying step at the film formation of a skin layer of the composite membrane. A single-component spunbond laminate nonwoven fabric free from a low melting-point component is also disclosed. However, in the case of a single component, delamination is liable to occur, causing a problem in the handleability, and although relatively high strength (5% elongational stress) is exhibited at the initial elongation stage based on the tensile stress, when a stress larger than that is applied, respective layers are easily separated and sequentially broken stepwise according to the breaking stress of each layer, as a result, the entire nonwoven fabric is disadvantageously broken under a low stress. In addition, similarly to International Publication No. 2006/068100 filed by the present inventors, a single-component laminate nonwoven fabric, where a meltblown nonwoven fabric is disposed between two spunbond nonwoven fabric layers, is also described, but this fabric has the same problem as that in International Publication No. 2006/068100. Also, as the joining method, thermal pressure-contact by calendering using a normal heated metal roll and a non-heated elastic roll containing a silicon-based resin roll, which is performed for the purpose of suppressing fusion of fibers on the filaments nonwoven fabric surface and keeping the fiber shape, is disclosed. In this regard, although thermal pressure-contact in two stages is usually applied so that the heated roll can contact with both the front and back surfaces, in the case of a single component, thermal pressure-contact by calendering in the second stage cannot be sufficiently effected, and this causes a problem that fluffing due to delamination or weak bonding of surface fibers is liable to occur. This is considered to result because crystallization of the nonwoven fabric proceeds by the calendering in the first stage.
Also, International Publication No. 2007/070688 (Japanese Unexamined Patent Publication No. 2009-519818) discloses an SMS-type separation membrane support composed of a laminate nonwoven fabric obtained by using, as surface layers, a spunbond layer having a core-sheath structure where a low melting-point component is used for the sheath part and a high melting-point component is used for the core part, and disposing a meltblown layer as an interlayer. However, a low melting-point component is melt-bonded to produce a locally-filmed site, as a result, uniformity of coating is not satisfied and moreover, dimensional stability in high-temperature processing is lacking.
Similarly, International Publication No. 2009/017086 has proposed to use, as the membrane support, a laminate nonwoven fabric that is composed of a spunbond nonwoven fabric having a core-sheath structure where a low melting-point component is used for the sheath part and a high melting-point component is used for the core part, but this technique is not sufficient in view of resinification of a low melting-point component and dimensional stability in high-temperature processing. Also, a support nonwoven fabric obtained by thermally pressure-contacting a single-layer spunbond web with a pair of steel rolls is disclosed as a support nonwoven fabric composed of a PET single component, which does not contain a low melting-point component and exhibits a tensile strength and a surface fiber bonding property necessary as a membrane support. However, because of a singe layer, non-uniformity ascribable to dispersion of the basis weight of spunbond, and filming due to excessive pressure-contact in a local high basis-weight portion are readily generated, and thus, the nonwoven fabric is unsatisfactory.